High-yield catalyst components for the polymerization of olefins such as ethylene can be obtained by supporting, on a magnesium dihalide, a titanium compound such as titanium halides, alkoxides and haloalcoholates. The catalyst components may be combined with an aluminum alkyl compound for the polymerization of ethylene. These catalyst components, and the catalysts obtained therefrom, are largely used in the plants for the (co)polymerization of ethylene both operating in liquid phase (slurry or bulk) and in gas-phase. However, the use of the catalyst components can be challenging due to the high reactivity of ethylene, which causes the kinetics of the polymerization reaction to be very high. As a consequence, the catalyst may be subject to tensile forces during the initial stage of polymerization which can cause the uncontrolled breakage of the catalyst, the resulting formation of fine polymer particles and the subsequent low bulk density of the polymer and operating process difficulties.
In the attempt to solve these problems, the catalyst may be pre-polymerized under controlled conditions to obtain pre-polymerized catalysts having good morphology. The catalyst may therefore increase its resistance during pre-polymerization such that the tendency to break under polymerization conditions is decreased. The formation of fine particles is therefore reduced and the overall bulk density improved. The catalysts may be prepolymerized with one olefin, which can be the same olefin, and then polymerized in the main polymerization step. Accordingly, the non-stereospecific catalysts used for ethylene polymerization can be pre-polymerized with ethylene. However, pre-polymerization with ethylene of a non-stereospecific catalyst to be used in ethylene polymerization can be problematic. For instance, the morphological properties and resistance of the catalyst may be improved only in combination with a decrease of the catalyst polymerization activity as described in U.S. Pat. No. 4,325,837. The activity and the morphological properties of the polymer produced with the pre-polymerized catalyst are in certain cases lower than that of the non-pre-polymerized one.
WO 01/85803 describes that pre-polymerization of a non-stereospecific catalyst with prochiral monomers it would be possible to obtain a catalyst for the polymerization of olefins capable to produce polymers with high bulk density and that has an activity which is higher than that of the original non pre-polymerized catalyst. The prepolymerization is carried out at room temperature, in the presence of an aluminum alkyl compound and monomer (propylene) so as to obtain low polymerization conversion. However, the resulting prepolymerized catalyst may need to be improved in terms of morphological stability when subject to ethylene polymerization carried out under drastic conditions. Moreover, the prepolymerized catalyst particles may show a slow settling velocity which decreases catalytic productivity.
The applicant has now found that when the pre-polymer of a prepolymerized catalyst shows specific features in terms of stereoregularity and molecular weight, the prepolymerized catalyst may be endowed with high settling velocity and morphological stability together with high activity. The catalyst may further generate high polymer yield polymers with regular morphology under drastic ethylene polymerization conditions.